1. Field of the Invention
The present invention relates to a feed line switching circuit for switching a feed line by energizing relays by feed current control, and more particularly to a feed line switching circuit suitable for a branching unit constituting an optical submarine cable system.
2. Description of Related Art
FIG. 7 is a circuit diagram showing a configuration of a conventional feed line switching circuit disclosed in Japanese patent application publication No. 7-34550/1995, for example. In FIG. 7, reference numerals 1a-1c each designate a feed line for feeding repeaters and the like with a current; 2a-2c each designate a power supply such as a DC constant current source; and the reference numeral 3 designates a ground.
The reference numeral 4 designates a feed line switching circuit for switching the feeding from the feeding across the feed lines 1a and 1b to the feeding across the feed lines 1a and 1c or vice versa by feed current control. In the feed line switching circuit 4, reference numerals 5a-5c designate terminals connected to the feed lines 1a-1c. 
The reference symbol K1 designates a relay connected between the feed lines 1a and 1b; K2 designates a relay connected between the feed lines 1a and 1c; reference numerals K3 and K4 designates relays connected in series across the feed lines 1b and 1c. The relays K1-K4 each consist of a vacuum relay or the like that operates in response to a feed current. The reference symbol k1-1 designates a contact set of the relay K1; k2-1 designates a contact set of the relay K2; reference symbols k3-1 and k3-2 each designate a contact set of the relay K3; and k4-1 and k4-2 each designate a contact set of the relay K4.
Next, the operation of the conventional feed line switching circuit will be described.
First, to achieve the feeding across the feed lines 1a and 1b, the power supplies 2a and 2b are driven so that the feed current flows from the power supply 2a to the power supply 2b through the feed line 1a, terminal 5a, relay K1, contact k4-2, contact set k2-1, terminal 5b and feed line 1b. As a result, repeaters (not shown) connected to the feed lines 1a and 1b are supplied by double-end feed. In this state, when the feed current reaches the working current of the relay K1, the relay K1 actuates its contact set k1-1 to open the connection across the feed lines 1a and 1c, and to close the connection between the relay K3 and the feed line 1c. Furthermore, the power supply 2c is driven to pass the feed current through the ground 3, relay K3, contact set k1-1, terminal 5c, feed line 1c and power supply 2c. As a result, the repeaters connected to the feed line 1c are supplied by single-end feed. When the feed current reaches the working current of the relay K3, the relay K3 closes its contact k3-1 and opens its contact k3-2, thereby self-holding the connection between the relay K3 and the feed line 1c. 
Likewise, to achieve the feeding across the feed lines 1a and 1c, the power supplies 2a and 2c are driven so that the feed current flows from the power supply 2a to the power supply 2c through the feed line 1a, terminal 5a, relay K2, contact k3-2, contact set k1-1, terminal 5c and feed line 1c. As a result, the repeaters connected to the feed lines 1a and 1c are supplied by double-end feed. In this state, when the feed current reaches the working current of the relay K2, the relay K2 actuates its contact set k2-1 to open the connection across the feed lines 1a and 1b, and to close the connection between the relay K4 and the feed line 1b. Furthermore, the power supply 2b is driven to pass the feed current through the ground 3, relay K4, contact set k1-1, terminal 5b, feed line 1b and power supply 2b. As a result, the repeaters connected to the feed line 1b are supplied by single-end feed. When the feed current reaches the working current of the relay K4, the relay K4 closes its contact k4-1 and opens its contact k4-2, thereby self-holding the connection between the relay K4 and the feed line 1b. 
In the conventional feed line switching circuit with the foregoing configuration, the impedance of the feed passage can sometimes vary because of the operation of the contacts, thereby varying the feed current flowing through the feed line switching circuit 4.
For example, assume that the feeding across the feed lines 1a and 1b is carried out by the feed current control when a breaking of the optical submarine cable on the side of the feed line 1c takes place, and a shunt fault occurs in which the end of the optical submarine cable on the side of the feed line 1c is grounded by seawater. In this case, the feed current passes not only through the passage of the power supply 2axe2x80x94relay K1xe2x80x94power supply 2b, but also through the shunt fault pointxe2x80x94relay K2xe2x80x94relay K1xe2x80x94power supply 2b transiently. When the sum of the two feed currents reach the working current of the relay K1, the relay K1 opens the contact set k1-1, thereby disconnecting the shunt fault point. Thus, at the time when the contact set k1-1 activates, the impedance of the feed passage seen from the feed line 1b to the feed line switching circuit 4 increases, so that the feed current flowing through the feed line switching circuit 4 is reduced transiently.
Thus, the feed current flowing through the feed line switching circuit 4 is reduced when the feed current reaches the working current of the relay K1 and the contact set k1-1 is opened. As a result, the feed current flowing through the relay K1 is reduced at the same time, and hence the relay K1 can return to unenergized state again. In such a case, the relay K1 will repeat the energized state and unenergized state (chattering), presenting a problem of disabling the switching of the feed line.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a feed line switching circuit and branching unit capable of positively switching the feed line with preventing the current reduction in the relay after it is energized, and hence capable of preventing its chattering.
According to a first aspect of the present invention, there is provided a feed line switching circuit for switching feeding by feed current control between feeding across a first feed line and a second feed line and feeding across the first feed line and a third feed line, the feed line switching circuit comprising: a relay that is connected between the first feed line and the second feed line, and operates in response to a feed current flowing through the relay; a first contact of the relay that closes when the relay is unenergized and opens when the relay is energized;, and a resistor that constitutes a series circuit with the first contact, which series circuit is connected in parallel with the relay, wherein a resistance r of the relay and a resistance R of the resistor are set to satisfy a relationship of
(R/(r+R))xc3x97Ia less than Ib
where Ia is a current value flowing through the second feed line immediately before operation of the relay, and Ib is a current value flowing through the second feed line immediately after the operation of the relay.
Here, the relay may further comprise a second contact that is connected between the first feed line and the third feed line, and that closes when the relay is unenergized and opens when the relay is energized.
Besides, each of the resistors may consist of any one of a single resistor, a combination of a plurality of resistors and an assemblage of other elements that provides resistance in its entirety.
According to a second aspect of the present invention, there is provided a feed line switching circuit for switching feeding by feed current control between feeding across a first feed line and a second feed line and feeding across the first feed line and a third feed line, the feed line switching circuit comprising: a relay that constitutes a series circuit with a first resistor, and operates in response to a feed current flowing through the relay, the series circuit being connected between the first feed line and the third feed line; a first switching section of the relay that closes its first and second contacts and opens its first and third contacts when the relay is unenergized, and that closes its first and third contacts and opens its first and second contacts when the relay is energized; a second switching section of the relay that is connected between the first feed line and the second feed line, and that closes when the relay is unenergized, and opens when the relay is energized; a first line, a first end of which is connected to a connecting point of the relay and the first resistor, and a second end of which is connected to the first contact of the first switching section of the relay; a second resistor, a first end of which is connected to a relay side end of the series circuit, and a second end of which is connected to the second contact of the first switching section of the relay; and a second line, a first end of which is connected to the first resistor side of the series circuit, and a second end of which is connected to the third contact of the first switching section of the relay, wherein a resistance r of the relay and a resistance R of the second resistor are set to satisfy a relationship of
(R/(r+R))xc3x97Ia less than Ib
where Ia is a current value flowing through the third feed line immediately before operation of the relay, and Ib is a current value flowing through the third feed line immediately after the operation of the relay.
Here, each of the resistors may consist of any one of a single resistor, a combination of a plurality of resistors and an assemblage of other elements that provides resistance in its entirety.
According to a third aspect of the present invention, there is provided a branching unit comprising a feed line switching circuit, the feed line switching circuit comprising: a relay that is connected between the first feed line and the second feed line, and operates in response to a feed current flowing through the relay; a first contact of the relay that closes when the relay is unenergized and opens when the relay is energized; and a resistor that constitutes a series circuit with the first contact, which series circuit is connected in parallel with the relay, wherein a resistance r of the relay and a resistance R of the resistor are set to satisfy a relationship of
(R/(r+R))xc3x97Ia less than Ib
where Ia is a current value flowing through the second feed line immediately before operation of the relay, and Ib is a current value flowing through the second feed line immediately after the operation of the relay.